In existing such systems, the bus length is limited to 6.0 meters and the stubs have a maximum length of 0.1 meters. The terminators each comprise a resistance divider formed by a pair of resistors, conveniently 220 ohms and 330 ohms, respectively. Hence a voltage of 4.0 volts applied to the terminator will give a "high" voltage of 2.4 volts on the bus, giving a margin of 400 millivolts over a "high" level threshold of 2.0 volts.
Waves propagate along the bus at approximately 200 meters per microsecond, so it takes less than 1 nanosecond for an incident wave to transit a 0.1 meter stub. This stub-transit time is much less than the typcial wavefront rise-time of 8 nanoseconds minimum, so the 0.1 meter stub behaves like a capacitor and reflections are not a serious problem.
In some applications, however, stub lengths up to 0.5 meters are preferred. An example of such an application is the DV-1.TM. system by Northern Telecom Limited employing several shared resource units. With 0.5 meter stubs, the transit time approaches the wavefront risetime and reflections become a matter of concern. Experiments have shown that for high-to-low transitions the reflections die out quickly and the receiver input voltages remain below the "low" level threshold specified in the afore-mentioned specification. On the other hand, low-to-high transitions often produce reflections which drive the receiver input voltages below the "high" levels. This is more likely to occur, of course, when the terminator bias voltage is at its "worst case" low value.
It is desirable, therefore, to maintain the terminator bias voltage at near the supply voltage, usually 5.0 volts, thus providing a margin large enough to tolerate reflections without the receiver input voltage being driven beyond the threshold.
Known current drivers for SCSI (Small Computer Standard Interface) systems, comprise a plurality of silicon diodes each connected to a corresponding one of a plurality of 5.0 volts sources. The diodes are connected, each by a fuse, in common to the terminator's power terminal. This arrangement provides individual overcurrent protection and source isolation.
Such known arrangements are not entirely satisfactory since the voltage drop across the diodes leaves insufficient margin for reflections. For example, for an applied voltage V.sub.CC of 4.8 volts, and a typical voltage drop across the diode of 800 millivolts, the terminator voltage would be only 4.0 volts.